How to store the row value from csv in python?

Question

I have a main program built in C++ that generates a CSV file. I've made an interface with PyQt5 and I want to read some specific columns from that CSV.

I can read but I want to store them to make a plot with matplotlib. Probably my fault is that I'm trying to work with the row like if it is an array. This is how my readfile function looks:

def readCSV(self):
    try:
        with open('resultado.csv') as csvFile:
            reader = csv.DictReader(csvFile, delimiter=';')
            i = 0
            x, y = []
            for row in reader:
                print(row["TOA (ns)"], row["Frecuencia Inicial (MHz)"])
                x[i] = row["TOA (ns)"]
                y[i] = row["Frecuencia Inicial (MHz)"]
                i = i+1
    except:
        print("Error: ", sys.exc_info()[0])

The exception prints: Error: class 'ValueError'

This are the 2 first lines of my csv, the next line are all float values. I only want TOA and Frecuencia Inicial columns:

The shown column got the Header Frecuencia Inicial without (MHz). This might also be a source for errors. But the exception is thrown somewhere else, as a wrong column name would throw a KeyError. Can you tell the line that's actually throwing the error? — Igl3
– Igl3, Commented Jul 13, 2018 at 9:32
x, y = [] will try to unpack an empty list into two variables. That raises ValueError: not enough values to unpack (expected 2, got 0) — shmee
– shmee, Commented Jul 13, 2018 at 9:41

hootnot · Accepted Answer · 2018-07-13 10:07:14Z

3

Some changes to your code: just append the values to the x and y array, use columnnames exact

 def readCSV():
     with open('resultado.csv') as csvFile:
        reader = csv.DictReader(csvFile, delimiter=';')
        x, y = [], [] 
        for row in reader:
            try:
                print(row["TOA (ns)"], row["Frecuencia Inicial"])
                # guess you want
                x.append(row["TOA (ns)"])
                y.append(row["Frecuencia Inicial"])
                #i = i+1
            except Exception as e:
                print("Error: ", e)

     # do something with x, y
     # since you use 'self' in the function def I asume this is
     # a class method, so you could make x and y class properties
     # and then use self.x and self.y in this code

     print(x)
     print(y)

You might want to checkout the use of Pandas:

import pandas as pd
df = pd.read_csv("/tmp/indata.csv", delimiter=";")
df
   TOA (ns)  Frecuencia Inicial  
0        10                2000   
1        20                3000

From a jupyter notebook you can do the plot with:

df[['TOA (ns)', 'Frecuencia Inicial']].plot(figsize=(20,10))

You can still finetune your plots using matplotlib and use data from a Pandas dataframe

edited Jul 13, 2018 at 10:07

answered Jul 13, 2018 at 9:35

hootnot

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2 Comments

shmee Over a year ago

That won't work, see my comment below the question regarding x, y = []

hootnot Over a year ago

@shmee : I've modified it

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